This invention relates generally to the edging of ophthalmic lenses and more particularly concerns a process for improving the bevel control operation of a lens edging machine. The control bevel operation of a lens edging machine controls the bevel placement around the lens shape. It is desirable to locate the bevel position equidistant in relation to the front curve of the lens so as to dispose the lens thickness on the wearer's side of the frame and thus preserve the frame's aesthetic quality.
Some presently known edging machines rely on mechanical forces in an attempt to maintain a consistent disposition of the edging wheel along the front curve of the lens. The edging wheel face is angled inwardly on either side of the bevel so that the wheel will be self-centered by the force of the lens within the angled face of the wheel. It is also imperative that the edging wheel floats freely in relation to the lens. However, the response of this relationship is not truly precise and, as the thickness of the lens increases, the wheel is no longer able to position itself properly.
Other edging machines attempt to overcome this problem by the use of patterns or servo-mechanisms which position the edging wheel either in response to the physical shape of the pattern or to the stored data which controls the operation of the servo system. However, present edging machines fail to place the bevel accurately because they do not use the correct information in deriving the horizontal displacement of the edging wheel. These systems use at best two dimensional data to control a three-dimensional function. Considering the axes of rotation of the edging wheel and the lens to be aligned in the X direction and a line normal to those axes and connecting them to be aligned in the Y direction, present systems typically use only Y axis data to determine the X axis displacement. However, because of the front curvature of the lens, the X displacement to the point of contact between the edging wheel and the lens is also a function of a vertical or Z axis distance taken from the actual point of contact between the lens and the edging wheel to the X-Y plane.
Considering the problem from another vantage point, present servo-type edging machines can only be accurate if the lens shape is circular, because the point of contact of the lens with the edging wheel must then be in the plane described by the axis of rotation of the lens shape and the center of rotation of the edging wheel. However, as the lens shape contorts to fit into fashion frames, the point of contact for various angular positions of the shape are not necessarily in that plane. As a result, the bevel thins out and the horizontal position "hooks" drastically at sharp lens corners.
Since the edging wheel has a finite non-zero radius, the edging machine must account for additional radial offset to prevent from cutting into the shape at a point of contact not in the plane connecting the centers of rotation of the lens and the wheel. The present process of computing the radial offsets Y or linear distances between the points of rotation of the lens and the wheel from the point data of the shape is called point-to-wheel data conversion. Point data, which is based on the radius of the shape, or wheel data, which is based on the displacement of the wheel from the shape, is inadequate to correct the problem.
It is therefore an object of this invention to provide an improved process for edging a lens which takes into consideration the curvature or X displacement of the lens edge in relation to a vertical or Z axis. Another object of this invention is to provide an improved process for edging a lens which employs servo-mechanism principles based on three-dimensional data. It is also an object of this invention to provide an improved process for edging a lens which enhances the consistency of displacement of the bevel of the lens in relation to the front face of the lens. And it is an object of this invention to provide an improved process for edging a lens which reduces the "hooking" manifested in lenses ground by edgers lacking three-dimensional capability.